1. Field of the Invention
The present invention generally relates to a reluctance machine operated as a generator. More particularly, embodiments of the present invention relate to the operation of a variable reluctance generator which is able to generate into a load without the use of active switches in its phase winding circuits.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM'93, Nürnberg, 21–24 Jun. 1993, incorporated herein by reference. FIG. 1(a) shows a typical switched reluctance drive in schematic form, where the switched reluctance machine 12 is connected to a load 19. The DC power supply 11 can be rectified and filtered AC mains or a battery or some other form of electrical storage. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the machine 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive, and a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The rotor position detector 15 may take many forms, including that of a software algorithm, and its output may also be used to generate a speed feedback signal. The presence of the position detector and the use of an excitation strategy which is completely dependent on the instantaneous position of the rotor leads to these machines having the generic description of “rotor position switched”.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. One of the most common configurations is shown for a single phase of a polyphase system in FIG. 2, in which the phase winding 16 of the machine is connected in series with two active switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A low-value resistor 28 is connected in series with the lower switch to act as a simple current transducer. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter.
FIG. 3(a)–3(c) show typical waveforms for two operating cycles of the circuit shown in FIG. 2 when the machine is in the motoring mode. FIG. 3(a) shows the voltage being applied at the “on angle” θon for the duration of the conduction angle θc when the active switches 21 and 22 are closed. FIG. 3(b) shows the current in the phase winding 16 rising to a peak and then falling slightly. At the end of the conduction period, the “off angle” θoff is reached, the switches are opened and the current transfers to the diodes, placing the inverted link voltage across the winding and hence forcing down the flux and the current to zero. At zero current, the diodes cease to conduct and the circuit is inactive until the start of a subsequent conduction period. The current on the DC link reverses when the switches are opened, as shown in FIG. 3(c), and the returned current represents energy being returned to the supply. The shape of the current waveform varies depending on the operating point of the machine and on the switching strategy adopted. As is well-known and described in, for example, the Stephenson paper cited above, low-speed operation generally involves the use of current chopping to contain the peak currents, and switching off the switches non-simultaneously gives an operating mode generally known as “freewheeling”.
As is well known in the art, switched reluctance machines can be operated in the generating mode. A typical arrangement is shown in FIG. 1(b), where the load 19 of FIG. 1(a) becomes the prime mover 19′, such as an internal combustion engine, supplying mechanical energy. The power supply 11 becomes an electrical load 11′, accepting energy from the electrical machine 12 through the power converter 13. In general, the phase currents are mirror images (in time) of the phase currents in the motoring mode. Such systems are discussed in, for example, “Generating with the switched reluctance motor”, Radun, Proceedings of the IEEE 9th Applied Power Electronics Conference, Orlando, Fla., 13–17 Feb. 1994, pp 41–47, incorporated herein by reference. FIG. 4(a) illustrates a flux waveform and the corresponding current waveform when the system is motoring and FIG. 4(b) illustrates the corresponding waveforms for generating. It will be seen from FIG. 4(b) that the machine requires a “priming” or magnetizing flux to be established (along with the necessary current to support this flux) before the energy is returned to the DC link. In other words, some electrical energy is required from the DC link to prime the machine before it is able to convert a larger amount of mechanical energy back to the DC link.
Though there are many topologies used for power converters for switched reluctance machines, all of them use a certain number of active switches, and these switches represent a significant portion of the cost of the converter. Considerable effort over many years has been put into reducing the number of switches per phase.